Removal of contaminants from water

ABSTRACT

At coal mining sites rain runoff can be laden with selenium, in the selenate form. The selenium-containing runoff water is caught in ponds. Remediation of the pond water to remove selenate, or remediation of other contaminated water with selenium or other contaminants, is performed by treatment with sulfur-modified iron (SMI) in a contact bed of an upflow reactor vessel. The contaminated water after pretreatment is pumped through the SMI reactor, where the SMI removes selenium and/or other contaminants from the water. For extending effectiveness and life of the SMI the contact bed is either periodically “fluffed” with a high-flowrate upflow of water or gas through the bed, or moved continuously through the column in a continuous medium filtration system. In either event, a gas inert to SMI, such as nitrogen, can be used in fluffing the SMI or lifting the SMI as a continuously moving medium.

This application is a continuation-in-part of application Ser. No.15/210,754, filed Jul. 14, 2016, now U.S. Pat. No. 9,878,922, which wasa continuation of application Ser. No. 15/058,090, filed Mar. 1, 2016,now U.S. Pat. No. 9,427,706, issued Aug. 30, 2016, which was acontinuation-in-part of application Ser. No. 14/170,472, filed Jan. 31,2014, now U.S. Pat. No. 9,272,934, issued Mar. 1, 2016, which was acontinuation-in-part of application Ser. No. 13/555,620, filed Jul. 23,2012 now abandoned, and also claimed benefit of provisional applicationSer. No. 61/759,039, filed Jan. 31, 2013.

BACKGROUND OF THE INVENTION

This invention concerns remediation of surface waters by removal oftoxic metals prior to discharge of the waters to streams and lakes. Inparticular the invention concerns overburden from surface coal mining incertain regions where selenium-containing rock is present in theoverburden, and where rainwater leaching through the rock introducesobjectionable levels of selenium into the runoff water, typically in theform of selenate, these levels being too high for discharge to streamsand lakes.

The subject of this invention is related to that of U.S. Pat. Nos.4,940,549, 5,200,082, 5,575,919, 5,866,014, 6,093,328 and 6,926,878. Inparticular, the latter four patents, U.S. Pat. Nos. 5,575,919,5,866,014, 6,093,328 and 6,926,878 disclose preparation of asulfur-modified iron premix (SMI) for use in treating water to removecertain substances. This invention utilizes a sulfur-modified iron (SMI)premix similar to that produced in accordance with the above patents, ina reaction column to treat runoff water laden with selenium, inparticular, selenate. All of the above patents are incorporated hereinby reference.

Surface coal mining operations produce large volumes of overburden,which is removed to reach coal seams. In the United States theoverburden must ultimately be replaced when the mining operation iscompleted or moves on, and the land is reclaimed essentially back to itsoriginal condition and topography. However, the overburden exposes amultitude of rocks to rainfall, i.e. to moving water which can leach therocks and take on substances, including minerals, present in the rocks.Rainwater is nearly pure and is a strong leaching agent. These rocks ifleft undisturbed would be subject to little or no rainwater leaching.The result is that runoff from rain can carry objectionable levels ofdissolved substances, leached from the exposed rock, into lakes andstreams.

Particularly in the Appalachian region of the United States many surfacecoal mining locations have rock and soil (“rock” herein) that containsselenium, especially in the selenate form. The removal of the overburdenexposes these rocks and even after reclamation, runoff from rainwater inmany cases exceeds permissible levels of selenium allowed for dischargeto lakes and streams. In many cases the runoff water, or much of it, hasbeen trapped in detention “ponds” at various locations around the siteof a surface mine, including after reclamation, pursuant to regulationsthat prohibit sediment-laden runoff water from being discharged to lakesand streams at excavation sites. However, eventually the pond water mustbe withdrawn or allowed to overflow, ultimately reaching lakes orstreams. Although sediment is much less, dissolved minerals remain,including selenium. There has been no economically feasible method forremediation of this selenium problem, and it affects many surface mines,particular in the Appalachian region, as well as others.

SUMMARY OF THE INVENTION

In the process of the invention, the selenium-containing runoff water ata surface coal mining site is caught in detention ponds. Remediation ofthe pond water to remove selenate down to permissible levels fordischarge to lakes and streams is performed by treatment withsulfur-modified iron (SMI) in an upflow reactor vessel. After aprefiltering step to remove suspended and colloidal solids, the pondwater is pH-adjusted as needed and pumped through the SMI reactor inupward flow, for a specified detention time in the SMI. The treatedwater, low in selenium, can then be run through an oxidation tank andfiltered to remove dissolved iron that is a residual from the SMIprocess. The process is run in continuous flow.

The system includes provision for “fluffing” the SMI mediumperiodically, to expand the bed of SMI with a rapid flow of waterthrough the medium. Not a backflush, this fluffing opens up the spacesbetween SMI particles, reversing compacting that has occurred, andrefreshes the active surfaces to extend the life of the bed. This can bedone once or more per day, for about ten to twenty minutes, at about tentimes normal (service) flow rate. Fluff flow is in the same upflowdirection as normal service flow. This fluffing cycle is useful in anySMI reactor column removing metals or other substances from water, notlimited to coal mine runoff.

In a second embodiment of the invention, the process is the same asdescribed above except that powdered iron is used instead of SMI. Thepowdered iron is a finely divided elemental iron (zero-valent), withparticle size preferably about 40-80 mesh (0.017 inch to 0.007 inch).Powdered iron will react with selenium, i.e. will adsorb selenium, butnot as efficiently as will SMI. In the reactor system and process of theinvention, the other steps of collecting runoff in a pond, prefiltering,adjustment of pH where necessary, and passing the water through anupflow reactor, followed by steps to remove iron oxide, and periodicfluffing of the contact bed medium, are the same as described above.

It is an object of the invention to provide a process and system whicheffectively and efficiently remove selenium from runoff water at a coalmining site, during mine operation and after mine site reclamation.These and other objects, advantages and features of the invention willbe apparent from the following description of a preferred embodiment,considered along with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view indicating steps of the invention, includingcollecting runoff from surface mining coal overburden in a pond,withdrawing the pond water and processing the water through the steps ofthe invention.

FIG. 2 is a similar view but showing a modified process.

FIG. 3 is a more detailed schematic elevation view of the reactor vesselof the system.

FIG. 3A is a schematic plan view showing elements of the reactor vessel.

FIG. 4 is a plan view showing a layout of service flow diffusers in thereactor vessel.

FIG. 5 is a detail view of a service flow diffuser.

FIG. 6 is a plan view showing a layout of fluff flow nozzles.

FIG. 7 is a detail view showing fluff nozzles.

FIG. 8 is a schematic elevation view in section, showing a continuousfiltration system as a reactor column.

FIG. 9 is a diagram similar to FIG. 1 but showing a modification.

DESCRIPTION OF PREFERRED EMBODIMENTS

Permits for surface mine operators in the U.S. require that the rainfallrunoff drainage from the surface mine site be controlled so that norunoff sediment is carried down into creeks, streams or lakes. Therainwater runoff flowing into these ponds has aggressively pulleddissolved minerals out of rocks, minerals such as sulfates (e.g. sodiumsulfate and magnesium sulfate) and selenium, typically in the selenateform, these minerals being highly soluble. The runoff ponds tend toconcentrate the dissolved minerals even further due to evaporation fromthe ponds. The selenate is difficult to remove from an aqueous solution.

FIG. 1 schematically shows important aspects of the invention. Runoffwater emanating from rainfall is indicated at 10, flowing over andthrough the overburden 12 from a surface coal mining operation,sometimes called strip mining. This overburden is replaced aftercompletion of mining at a particular location, and although the land isreclaimed, and the topography essentially restored using the overburdensoil and rocks, this material has nonetheless been disturbed, and runoffrainwater, essentially pure and without dissolved minerals, willactively leach out some of the minerals from the rocks of theoverburden. This occurs both during mining and after reclamation.

As noted above, one particularly deleterious mineral typically leachedfrom rocks in certain geographical areas is selenium, usually in theform of selenate. Examples include compounds of selenium, includingselenate, sodium selenate, calcium selenate, magnesium selenate andvarious forms of selenides. The selenate ion is SeO₄ ²⁻.

Although selenium is a mineral needed in the diet of humans in verysmall quantities, it has toxic effects to aquatic life at higherconcentrations. Certain standards for discharge of runoff water fromdisturbed or reclaimed land place a limit of five parts per billion(ppb) selenium content for water that will be allowed to flow intostreams and lakes. This is imposed whenever human activity isresponsible for concentrating the level of selenium. In contrast, runoffwater which concentrates in runoff-catching ponds at a surface miningcoal site can have twenty to thirty parts per billion selenium andoccasionally more.

FIG. 1 indicates one such runoff-catching pond, at 14. By the processand system of the invention, the runoff pond water 14 is remediated toremove much of the selenium content, down to below the limit of 5 ppb.As indicated, the runoff water is pumped by a pump 16 from the pond(preferably a submerged pump suspended just below the surface), andprefiltered at 18 to remove suspended and colloidal solids. This can be,e.g., a sand filter. Following this filtration the water optionally canbe put through a carbon filter as indicated at 19 to remove anyadditional colloidal matter and any color left in the water.

The acidity of the filtered water is checked (and preferably isconstantly monitored) at a tank 20, and if needed, pH is adjusted inthis tank, which can be on a continuous-flow basis. An automatic systemthat monitors pH of the pond water and automatically adds acid for pHcorrection may be included in the tank or chamber 20. The preferredrange of pH for treatment in the invented process is below pH 7, andpreferably in the range of about 5.0 to about 7.0, more preferably about5.5 to about 6.5, most preferably about 6.0 to about 6.5. At a pHgreater than about 8, calcium can be precipitated during the treatment,which has a negative effect on treatment. In one preferred embodimentthe acidity of the water exiting the adjustment tank or zone 20 is aboutpH 6. This usually requires addition of acid (sulfuric acid in apreferred embodiment) in the tank or zone 20, since the runoff water ata location such as the Appalachian Mountains is typically in the rangeof about pH 7 to 7.5. Since the chemical reactions taking place in theSMI reactor tend to increase the pH to above 8.0, the acid addition isrequired to avoid impairing the life of the SMI.

The prefiltered runoff water, at a pH in the desired range, proceeds toa reactor column 22, which may involve another pump (after flow througha valve 25, the purpose of which will be explained below). The column 22comprises a vertical reactor vessel through which the water is pumped toflow preferably from bottom to top as indicated; upflow assures properand even contact with the reactant and helps keep the mass of reactantparticles “open” rather than compacted. The vessel is filled withsulfur-modified iron or SMI as noted above, the SMI being chemicallyreactive to remove selenate and some other materials from water byadsorption, as discussed in the patents cited above, incorporated hereinby reference. The depth of SMI in the reactor vessel should be limited,preferably no more than about six feet high (and preferably lower), toavoid compaction of the SMI particles. Openness is needed for bestcontact and treatment. In a reactor column having a diameter of about2.0 feet and a SMI bed depth of about 40 inches, the water will takeabout eight minutes to flow through the SMI bed (and additional time toexit at the top of the vessel). This is a preferred dwell time forcontact with the SMI, or preferably a range of about 5 to 15 minutes.The water flows out through a line 26 and a valve 27, explained below.

In a practical installation, a reactor vessel should have an insidediameter in a range of about 3 to 6 feet.

In the effluent of the reactor, indicated at the top of the reactor at26, the treated water is low in selenium, below permissible limits, andwill contain some calcium, magnesium and other minerals that may bepicked up via leaching, but the water also carries some additionaldissolved iron taken on from the SMI.

Iron can be removed from the treated effluent using oxidation andfiltration. For example, in a tank 28, the treated water can besubjected to bubbling aeration to oxidize iron in the water, creating aniron oxide precipitate. Oxidation can also be effected by chlorinationin a tank such as shown at 28.

FIG. 1 shows a post-filtration step 30, in which iron oxide precipitateis filtered out of the treated water, which can be with another sandfilter or another type of filter. The iron content is reduced preferablyto below 0.5 parts per million. As indicated in the drawing at 32, thetreated water after the sand filter can then be discharged to lakes orstreams.

The entire process of the invention can be, and preferably is, carriedout at atmospheric pressure (disregarding slight hydraulic pressure frompumping and in the reactor column).

As mentioned above, periodically the reactor column, i.e. the contactbed within the reactor 22, is subjected to “fluffing” to loosen and openthe bed to prevent or break up compacting that has occurred, to extendthe life of the medium. In FIG. 1 a separate flow circuit or fluff loopis indicated for this purpose, with the valve 23, a line 42, a holdingtank 34, a fluff pump 36, fluff nozzles 38 in the tank 22, and a fluffreturn line 40. Fluffing of the reactor contact bed is performedpreferably at least once daily, for about ten to twenty minutes. For afluff cycle the tank 34 is pre-filled. The valve 23, which receives theprefiltered, pH-adjusted pond water, is opened to allow water to flowthrough the line 42 to the holding tank, which is vented to allowfilling. Note that the line 42 could come from farther upstream, butpreferably is as shown, so that filtered water at desired acidity isused for fluffing the SMI. When a fluff cycle is begun the pump 24 andthe valves 25 and 27 are shut off and the fluff pump 36 is activated,drawing water out of the already-filled holding tank 34 and deliveringthe water at high velocity, approximately ten times or more service flowrate, through the fluff nozzles 38 in the tank. This rapid flow,preferably directed downwardly as it exits the nozzles 38, stirs up,fluffs, expands and fluidizes the SMI reactor bed, normally expandingthe bed to twice its normal volume in the tank. With the service flowexit valve 27 closed, fluff water exits the tank at 40 and can bereturned to the pond water 14. This water could be directed in adifferent way if desired, such as to the holding tank 34, although thewater, still with high selenium, would also carry an elevated ironcontent from contact with the medium and thus return of the water to therunoff pond is generally preferable.

During the fluff cycle the valve 23 can remain open, slowly admittingwater into the holding tank 34 while this tank is essentially drained bythe fluff pump 36. The holding tank provides storage for the surge offluff water flow that occurs during the fluff cycle. Note that the tank34 is vented.

At the end of a fluff cycle, the valve 27 is opened, the valve 25 isopened to deliver service water flow via the pump 24, which isreactivated, and the fluff pump 36 is shut off. However, the holdingtank 34 must be refilled, which can be done simultaneously with normalservice flow through the reactor, and the valve 23 remains open (or isreopened if it has been closed during fluff). FIG. 1 is a simplifiedview, and in many instances there will be a plurality of reactors 22,e.g. ten or more, operating in parallel. The valve 25 is a schematicindication, and typically the pump 24 will provide service flow to aseries of reactor vessels, e.g. ten or more, or each vessel can have itsown service pump. The fluff flow is a much higher rate of flow, thus theneed for the holding tank 34. The tank 34 can be open at top orotherwise vented so that it can be essentially drained during fluff. Itcan then be refilled at a slower rate, from the water flow downstream ofthe pH adjustment tank. Normally one reactor tank 22 is fluffed at atime.

As indicated in FIG. 1 at 43, a biocide may be added to the water in theholding tank 34, or into the line leading to the fluff nozzles 38, tointroduce the biocide into the fluffing or fluidizing water. This can beimportant because the standing water in the runoff pond 14 tends to pickup a biofilm that can grow on and reduce effectiveness of the SMI. Evenif the fluffing were not included in the system, the biocide could beintroduced continually in an inflow conduit such as the line leading tothe fluff nozzles. Glutaraldehyde and DBNPA can be particularlyeffective biocides. The addition of biodispersants can also helppenetrate biofilms. DBNPA or 2,2-dibromo-3-nitrilopropionamide is aquick-kill biocide. Glutaraldehyde is an organic compound with theformula CH₂(CH₂CHO)₂. A pungent colorless oily liquid, glutarldehyde isused to sterilize medical and dental equipment.

Both chemical biocides have been used and found effective to reduce themetabolic process of the biofilm that can grow on sulfur modified iron(SMI). Either can be added into the water that is used to fluff orfluidize the SMI bed. An effective concentration is about 10 to 30 mgper liter of fluff water, added by batch to the holding tank orcontinuously into a pipe leading to the treatment column. The compoundsbreak down into harmless elements and can be discharged with the streamthat has been treated.

The reactor 22 is shown in greater detail in FIGS. 3 and 3A, where theschematic views essentially show the reactor vessel in cross section,both elevation and top plan. FIG. 3 shows the service pump 24, alsodesigned as P, directing the prefiltered and pH-adjusted pond water intothe bottom of the tank via a header or manifold pipe 45 at the tankbottom. The header 45, preferably a stainless steel pipe of about 4 inchdiameter, is shown in FIG. 4 as feeding a series of diffuser tubes 46 ofvarious lengths in accordance with the circular shape of the reactor asindicated at 22. Couplings 47 secure the tubes 46 to the header. Thesediffusers evenly disperse the pond water around the area of the reactor.The diffuser tubes preferably are stainless steel pipe, e.g. ¾ inch 316stainless steel pipe, although they could be epoxy coated carbon steel.These pipes are plugged at the ends and have diffuser holes, which maybe about ⅛ inch diameter, essentially equally spaced on the diffuserpipes and numbering about 68 (about 60 to 75) in a reactor tank havingan outside diameter of 44 inches, inside diameter slightly less (about43 inches). A range of diameter for practical purposes is about 3 feetto 6 feet. These pipes preferably are each encapsulated with acylindrical screen 48 as indicated in FIG. 4. The screens help diffusethe pond water more evenly within the tank, and they prevent the reactormedium from clogging the delivery holes of the pipe. The screens areadvantageously made of wedge wire, triangular in cross section, withslots facing inwardly toward the diffuser pipe. The screen slots may beabout 0.005 inch wide, with the diffuser screen cylinder beingapproximately 2⅜ inch O.D. For example, these stainless steel screensmay be made of wedge wire such as made by Johnson Screens(johnsonscreens.com) and often used in water wells. The screens havetheir slits facing inwardly.

FIG. 3 indicates the reactor medium 50 in the tank 22, the mediumextending through less than half the height of the tank, and typicallyonly about ⅓ the height of the tank. Also shown in FIG. 3 is the fluffpump 36, feeding high-velocity fluff water to the tank via a header 52spaced above the service header 45. As explained above, the fluff pump36 is active when the service pump 24 is inactive, and vice versa. Thefluff header 52 is spaced a short distance above the header 45, e.g.about 4 inches, with short drop pipes 54 that extend down to deliverfluff water at approximately the same level as the service deliverypipes, i.e. as close to the bottom of the reactor vessel as practicable.

FIG. 6 shows one preferred layout for the fluff water delivery system.The fluff header 52 can be a 4 inch pipe, to which are secured a seriesof laterals 56, with lengths that vary in accordance with the circularlayout, as shown. There may be fewer of these than in the case of theservice flow diffusers, as well as fewer and larger exit orifices, fordelivery of the fluff flow which is approximately ten times or more thevolumetric flow rate of the service flow.

As shown in FIGS. 6 and 7, the 4 inch header pipe 52 delivers the fluffflow via the laterals 56 down through the drop pipes 54 to fluff nozzleswhich, like the service nozzles, preferably are enclosed withincylindrical wedge wire screens 58. The drop pipe ends 54 a enclosedwithin the screen preferably comprise, on each drop pipe, about fourholes of about ⅜ inch diameter. Thus, in the layout shown in FIGS. 5 and6 there are preferably about 48 holes. In a preferred embodiment thelaterals 56 are 1½ inch pipe (preferably stainless steel), with the droppipes preferably 1 inch pipe. Ells 58 and tees 60 are shown connectingthe drop pipes to the lateral pipes 56, and these also are preferablystainless steel. Couplings are shown at 62 connecting the laterals tothe 4 inch header pipe 52, also preferably of stainless steel. Thecouplings are welded onto the header.

The drop pipes 54 are of a length to place the high-flow fluff exitnozzles near the bottom of the reactor tank, and this may be inessentially the same level as the service flow diffusers as describedabove.

The headers 52 and 45 in one preferred embodiment extend through thewalls of the reactor tank 22 as indicated in the drawings, and arewelded to the walls in sealed relationship.

As noted above, the contact bed 50 of SMI medium is expanded greatlyduring the fluffing cycle, such that it occupies usually twice or moreits normal volume in the tank. During the fluff cycle the fluffing waterexits the tank via an overflow weir 65. Water level during fluff isindicated at 66 in FIG. 3, higher than service flow level. The waterexits through a pipe 40, preferably a 6 inch pipe, which is the exitline schematically shown in FIG. 2, and this may return the fluff waterto the detention pond.

In normal, service flow, the water level will be approximately at theindicated level 70 in FIG. 3, with the water exiting through a pipe 72,which can be a 3 inch pipe. This feeds the exit line 26 indicated inFIGS. 1 and 2, with the valve 27 in this line to be shut off during afluff cycle. The pipe 72 has an input end 72 a at a level high enough inthe tank that the medium will not reach this level during a fluff cycle.

Chemical Reactions

As noted above, the SMI acts by adsorption. What appears to happen isthat the selenate ion is pulled apart. The selenium of the selenate ionis believed to combine partly with the sulfur and partly with the ironand iron oxide (and possibly iron hydroxide) of the SMI; bothselenium-sulfur and selenium-iron compounds exist in nature, usuallytogether with other mineral elements and oxides. In fact, powdered ironalone has been used in the prior art to remove selenium, but as comparedto the process with SMI, the use of powdered iron requires about ten toone hundred times more iron than does the SMI process. Also, many timesmore iron becomes dissolved in the treated water with the use ofpowdered iron rather than SMI.

The SMI in a reactor vessel is expected to remain active and effectivefor one year or more, when the reactor is operated at about 4-5 gallonsper minute per square foot SMI. When the SMI has lost most of itseffectiveness, it is replaced and the spent SMI can be melted down toretrieve the iron for re-use. Selenium content is minimal and can bedischarged to a stack house filter on the stack of the steel mill.

EXAMPLE 1

An Example of the Actual Operation of a Small Scale Reactor

Adjacent to a sediment pond below a remediated surface mine slope insouthern West Virginia.

A small scale SMI reactor was operated. It consisted of an 8 inchdiameter 10 foot high steel pipe with a flanged bottom and an open top.A 6 inch long, 1½ inch diameter wedge wire pipe was placed inside thepipe at the bottom of the reactor perpendicular to the pipe wall to actas a diffuser for the influent pond water. Sulfur modified iron (SMI)was put into the pipe to a depth of 40 inches and water passed upflowthrough the SMI bed at a rate of 0.6 gallons per minute. The time ofcontact of water with the bed was approximately 14 minutes and the flowflux was about 2 gallons per minute per square foot.

The influent water was adjusted in a separate tank with sulfuric acid toreduce the inlet pH from 7.4 to a pH of 6.0 as the water entered thereactor. The effluent pH from the reactor tank was pH 7.6.

The selenate concentration in the pond was approximately 30 parts perbillion and the selenate remaining in the water exiting the reactor was0.05 parts per billion. There was no iron detected in the influent waterand the iron concentration in the water at the outlet of the reactor was39 parts per million (which in practice would be reduced to below 0.5parts per million, as above).

The pilot was operated for approximately 30 days and at no time did theeffluent water concentration of selenate from the reactor exceed 1 partper billion.

In a second implementation of the invention, powdered iron is usedrather than SMI, in essentially the same process. As noted above,powdered iron has been used previously to remove selenium, but not inthe processes outlined above. FIG. 2 shows the modified process, withpowdered iron used in the reactor rather than SMI. The other steps aresimilar, including the steps to remove iron oxide from the waterpost-reaction, and also including the fluff cycle for periodicallyloosening and extending the life of the contact bed.

In the iron reaction, as noted above, greater quantities of iron areconsumed. The retention time in the reactor will be different, somewhatlonger, estimated from 5 minutes to 30 minutes. Flow rate will beslower, in a range of about 1-5 gallons per minute per square foot.

FIG. 8 shows an alternative to the fluffing cycle and subsystemdescribed above. In lieu of the fluffing cycle shown in FIG. 1 at 42,34, 36, 38, etc., a DYNASAND type continuous sand filter, i.e.continuously moving medium filter, such as shown in FIG. 8 is employedas the SMI reactor. Thus, in FIG. 1 the fluffing system is eliminatedand the SMI reactor is configured as shown at 22 a in FIG. 8. DYNASANDfilter systems are well known in the filtration industry. Herein thattype of system is referred to as a continuously moving medium filtrationsystem. Here, the DYNASAND type continuous medium filter is modified.SMI, indicated at 75, is placed in the upflow column rather than sand.As is well known, a continuously moving medium filter system generallyof the type shown in FIG. 8 continually moves the medium within thecolumn, by lifting medium through a central upflow column 76 to bereleased above, as shown by an arrow at 78, so that medium particlescontinually move down. In a sand filter, this continuous movement of thesand medium keeps the medium particles loose and open and not compacted.Some of these continuously moving medium filtration systems include amedium cleaner at the upper end of the column 76, but that is not thecase with the SMI column 22 a. In addition, the column 22 a does not useair to lift the medium, but instead uses water. As is also known, thecontinuously moving filtration system need not provide movement that iscontinuous in time; it can be intermittent, at such frequency andduration as to be effective in cleaning the medium adequately.

In FIG. 8, input runoff water (which can be from the point 24 in FIG. 1)enters at 80, flowing down an input conduit 82 and through an outerconduit 84 that is concentric over the medium lift column 76. The waterenters the contact bed at a low point at 86, as shown by arrows. Aportion of the input water (prior to the inlet at 80) is directed to thebottom inlet of the lifting column 76, as indicated 88. This is a smallportion of the influent runoff water; it can be recirculated out the topof the column as indicated at 90, back into the runoff water at anappropriate location, such as downstream of the pH adjusting tank 20 inFIG. 1.

As the water rises through the SMI contact bed 75 and exits the reactorcolumn at 92 via an overflow weir 93, the SMI medium is continuouslymoving downward, the granules migrating down around a cone 94 andeventually to the bottom of the conical lower end 96 of the reactorcolumn. The input water 88, which can be directed in a narrow jet streaminto the upflow conduit 76, draws SMI particles with it and up throughthe lift column 76. With the water at lower velocity at or near the topend 98 of the uplift column, the particles of medium are dropped out andfall back into the contact bed at the top 99 of the bed. Water flowthrough the column 76 can be at a low flow rate, such that the SMI bedis fully recirculated over an extended period, such as 8 to 16 hours ormore.

In this way, the SMI medium is constantly being moved as thecontinuously moving media operates, with particles tumbling andshuffling relative to one another, maintaining the bed somewhatfluidized, uncompacted and relatively open for promoting reaction of theSMI with selenium or other impurities. The SMI bed is more effective fora longer period of time.

It should be understood that the SMI reactor in the form of acontinuously moving medium filtration system can be used in otherapplications where selenium or other impurities are efficiently removedusing SMI. Examples are refinery water, heavy metal-contaminated groundwater, water with hexavalent chrome and otherwise contaminated waterused in industry or for domestic water.

As noted above, if selenium is the contaminant, references to seleniuminclude all contaminating compounds of selenium, including selenate,selenide, selenocyanate and organic compounds of selenium.

The system and process of the invention, in another embodiment, use notwater but inert gas to prevent compacting and to maintain looseness inthe SMI contact bed. This can be any gas inert to SMI, i.e. not reactivewith SMI, but the preferred gas is nitrogen. Nitrogen can be readily andinexpensively generated from air, using the pressure swing adsorptionsystem (PSA system). PSA is well known, and is explained, for example,online at american-environmental.us/pressure swing adsorption systems.PSA systems can produce nearly-pure nitrogen gas economically, e.g. 98%to 99%, which is adequate, and even higher purity is attainable.References to nitrogen gas herein are intended to include nearly-purenitrogen gas, at a level such that the SMI life is increased bypreventing or alleviating compaction.

In the case a continuously moving medium system is employed, the inertgas (meaning inert as to SMI) is directed up through the central upliftcolumn at 88 in FIG. 8, which indicates the alternative of inert gasrather than water. The gas lifts the particles of SMI, through theuplift column causing movement throughout the bed of SMI particles, andthe gas is discharged at 90 as shown in FIG. 8. The movement iscontinuous throughout the SMI bed, but need not be perpetuallycontinuous in time. A cycle might occur several times per day, even 10or 12 or more times per day, with cycle frequency and duration selectedso as to prevent compacting and maintain looseness in the SMI bed, tothe extent effective to achieve a desired result of prolonging theeffectiveness of the SMI in the bed. Continuous moving medium systemshave typically used air as the uplifting fluid. This cannot be done withSMI; deleterious reaction of oxygen with the SMI will occur. Efficientlygenerated nitrogen gas is an inexpensive and efficient alternative toair or water.

As explained above, the continuous moving medium system of FIG. 8 is analternative to the SMI fluffing subsystem described above and shown inFIGS. 1-3, 6 and 7. The fluffing cycle can be improved and made moreefficient using an inert gas rather than water. FIG. 9 shows a systemsimilar to FIG. 1 but with a modified form of fluffing achieved using agas inert to SMI, preferably nitrogen gas. Note that in FIG. 9, the SMIwater treatment system should be understood as applying to anycontaminated water containing selenium, or contaminated water with othercontaminants that are susceptible to removal using SMI. Thus, the coalmining runoff pond 14 as a source is only one example of theapplicability of the invention.

In the modified system and method depicted in FIG. 9, a pressure swingadsorption (PSA) device 100 is used to generate nitrogen, substantiallyall nitrogen gas, as indicated at 102. The PSA apparatus 100 is activeonly during a fluffing cycle, which may occur at periods as discussedabove (although the gas could be compressed and stored, then used asneeded). The produced nitrogen gas is compressed using a compressorindicated at 104 and denoted “C”. During a fluffing cycle, the servicepump 24 continues to pump contaminated water into the reactor columnthrough the diffusers 45. The rate of flow of the water is preferablyincreased, e.g. by 10% to 30%, still within the capability of theservice pump 24. Note that the special fluff delivery nozzles 38 ofFIGS. 1 and 2 are not required.

During fluffing the PSA device 100 is activated, producing nitrogen thatis compressed at 104 to pressure that may be approximately equal to orsomewhat higher than the water pressure downstream of the pump 24. Thegas at high volumetric flowrate enters the input water flow at 106. Thewater and compressed nitrogen together reach the diffusers 45, where thegas rapidly expands and is released as bubbles along with the fluffwater in a very high and turbulent fluid flowrate, so as to expand andloosen the contact bed to prevent or break up compacting of the SMImedium.

With the use of compressed, inert gas rather than fluff water, theholding tank of FIG. 1, the fluff pump 36 and the special piping 54,drop pipes and nozzles are eliminated. In addition, the return pipe 40for fluff water is eliminated. Considerable capital cost reduction isachieved, and the fluffing cycle is effective and efficient. As FIG. 9indicates, nitrogen gas is simply released back to the atmosphere,indicated at 108.

As noted above, contaminants referred to herein include any watercontaminants that are susceptible to removing using SMI. These mayinclude, in addition to selenium compounds, arsenic, technetium,vanadium, uranium, hexavalent chrome and other deleterious compounds.

The above described preferred embodiments are intended to illustrate theprinciples of the invention, but not to limit its scope. Otherembodiments and variations to these preferred embodiments will beapparent to those skilled in the art and may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

I claim:
 1. A method for removal of contaminants from contaminatedwater, comprising: prefiltering the contaminated water to removeparticulate material, if suspended particulate material is present,assuring pH of the prefiltered contaminated water is within a range ofabout pH 5.0 to about pH 7.0, and adjusting pH of the contaminated waterwhen necessary, pumping the contaminated water in upflow directionthrough a reactor column, in a vessel containing a contact bed of SMIwhich the contaminated water contacts, thereby removing a significantportion of dissolved contaminants from the water, to produce a treatedwater, and the reactor column being a continuously moving medium whereinthe medium comprises SMI, with the SMI in a recirculating path,migrating down in a mass by gravity in the reactor column to a bottomwherein particles of SMI are raised upward through an uplift column andthen deposited back onto the top of the contact bed, thereby preventingcompacting and maintaining looseness in the SMI bed.
 2. The method ofclaim 1, wherein the contaminants include selenium, which is removed bythe contact bed of SMI.
 3. The method of claim 2, wherein thecontaminated water is refinery water.
 4. The method of claim 2, whereinthe contaminated water is runoff water from overburden of surface coalmining.
 5. The method of claim 1, wherein the contaminated water isheavy metal-contaminated ground water.
 6. A system for removing seleniumfrom contaminated water used in industry or for domestic water,comprising: a treatment plant receiving a flow of said contaminatedwater and for removing selenium from the contaminated water, thetreatment plant including a reactor column receiving the contaminatedwater, the reactor column comprising a vessel containing a contact bedof SMI for contact with the contaminated water, and a pump for passingthe contaminated water through the reactor column in an upflowdirection, the reactor column being effective to remove a significantportion of selenium contained in the contaminated water to produce atreated water, and wherein the reactor column is a continuously movingmedium wherein the medium comprises SMI, with the SMI in a recirculatingpath, moving down by gravity in the reactor column to a bottom whereinparticles of SMI are raised upward through an uplift column and thendeposited back onto the top of the contact bed, thereby preventingcompacting and maintaining looseness in the SMI bed.